Dozing device for bulldozer

ABSTRACT

A dozing system for a bulldozer capable of providing high operational efficiency in dozing operation and a smooth excavation face. If it is determined when operation is performed in an automatic digging mode that the load exerted on the blade is stable, a target position for the cutting edge relative to the ground is corrected to the actual position of the cutting edge at that time. According to the ratio of the amount of excavated soil loaded on the front surface of the blade to the loading capacity of the blade front surface and/or the stability of the load exerted on the blade, a switching is performed between a weight characteristic for the operation amount of the load control and a weight characteristic for the operation amount of the smoothing control. Further, a map for correlating actual travel distance with the position of the blade cutting edge is prepared, and stable cutting edge positions are accumulated in each respective cycle and averaged to obtain an optimum target value for the smoothing control.

TECHNICAL FIELD

The present invention relates to a dozing system well suited for use ina bulldozer and more particularly to a leveling control technique foradequately controlling the position of the cutting edge of the blade inrelation to the ground during the dozing operation of a bulldozer.

BACKGROUND ART

Normally, the dozing operation of a bulldozer of the above type iscarried out under manual control by the operator. Concretely, the bladeis manually controlled so that digging or soil carrying is performedwith the blade being lifted and lowered, or leveling is performed withthe cutting edge of the blade kept in a certain position in relation tothe ground.

In such manual operation to lift or lower the blade, or keep theposition of the cutting edge, the operator is required to frequentlymanipulate the blade so that he gets tremendous fatigue, no matter howskillful he is. In addition, such manipulation is too complicated for aninexperienced operator.

As an attempt to solve this problem, the applicant of the presentinvention has proposed a leveling control system for a bulldozer inJapanese Patent Publication (KOKAI) No. 7-48855 (1995), which enablesleveling work in dozing operation by simple manipulation without causingextreme fatigue. In this leveling control system, a lift operationamount is obtained from a load control characteristic map to make theactual tractive force of the bulldozer equal to a target tractive force,while a lift operation amount is obtained from a leveling control(smoothing control) characteristic map to make the actual position ofthe cutting edge relative to the ground coincident with a target cuttingedge position. These lift operation amounts are respectively weightedwith a value obtained from a load-leveling control weight characteristicmap, based on the difference between the actual and target tractiveforces and then summed, in order that a final lift operation amount isobtained.

The leveling control system of this publication, however, presents thefollowing problem. In this system, even when the load exerted on theblade is greatly changed, a target value for the load control iscorrected by a target value for the smoothing control. Therefore, uponcompletion of carrying operation for example, the load control is soperformed as to lift the blade, whereas the smoothing control is soperformed as to lower the blade to restrict the fluctuation of thetarget cutting edge position. Consequently, the resultant ground surfaceafter dozing operation will be undulated.

In addition, according to the publication, the load-leveling controlweighting characteristic map is always set based on a constant weightfunction notwithstanding changes in the working states of dozingoperation, and therefore the weight function of such a map is inevitablya combination of a weight function for digging work and a weightfunction for carrying work. This poses an obstacle to improvements incontrol performance.

The above publication has a further disadvantage in that when performingdigging work and carrying work a plurality of times in the same lane, atarget value is reset for every cycle of dozing operation so thatimprovement cannot be expected from the repetitive cycles andconsequently there remain difficulties in adjusting the dozing operationto soil property and working conditions which vary every excavationsite.

The invention is directed to overcoming the above problems and a primeobject of the invention is therefore to provide a dozing system for abulldozer, which provides improved operational efficiency for dozingoperation while achieving a smooth excavation face. A second object ofthe invention is to provide a dozing system for a bulldozer, which iscapable of adequately setting a weight function according to whether adigging mode or carrying mode is presently selected, thereby achievingbetter control performance. A third object of the invention is toprovide a dozing system for a bulldozer, which exhibits goodconformability to variations in the conditions of every excavation siteto thereby achieve improved operational efficiency.

DISCLOSURE OF THE INVENTION

The first object can be achieved by a dozing system for use in abulldozer according to a first aspect of the invention, the dozingsystem comprising:

(a) cutting edge position detecting means for detecting the position ofthe cutting edge of a blade in relation to the ground;

(b) target cutting edge position setting means for setting a targetposition of the cutting edge of the blade in relation to the ground;

(c) load condition detecting means for determining whether the loadexerted on the blade is in a stable state;

(d) target cutting edge position correcting means for correcting thetarget cutting edge position set by the target cutting edge positionsetting means to the actual position of the cutting edge at that time,if the load condition detecting means determines that the load exertedon the blade is in a stable state when dozing operation is performed inan automatic digging mode; and

(e) blade controlling means for controlling the blade to be lifted orlowered such that the position of the cutting edge of the blade detectedby the cutting edge position detecting means is made coincident with thetarget cutting edge position corrected by the target cutting edgeposition correcting means.

According to the first aspect of the invention, if it is determined whendozing operation is carried out in an automatic digging mode that theload exerted on the blade is in a stable state, in other words, ifautomatic digging is stably carried out, a target position of thecutting edge of the blade is corrected to the actual position of thecutting edge at that time. According to this corrected target cuttingedge position, the control (smoothing control) is carried out foradjusting the position of the cutting edge of the blade in relation tothe ground. With this arrangement, the blade control can be accuratelycarried out and improved efficiency can be achieved. In addition,automatic digging can be so performed as to flatten the face of theexcavation, and it becomes possible to cope with variations in theinclination of the land and with the ground surface having irregularhardness.

In the invention, the actual position of the cutting edge used forcorrecting a target cutting edge position by the target cutting edgeposition correcting means is preferably obtained from a moving average.This enables high accuracy control.

The second object of the invention can be achieved by a dozing systemfor use in a bulldozer, according to the second aspect of the invention,the dozing system comprising:

(a) actual tractive force detecting means for detecting the actualtractive force of a vehicle body;

(b) cutting edge position detecting means for detecting the position ofthe cutting edge of a blade in relation to the ground;

(c) loading ratio detecting means for detecting a loading ratio that isthe ratio of the amount of excavated soil loaded on the front surface ofthe blade to the loading capacity of the blade front surface;

(d) first operation amount calculating means for calculating anoperating amount for controlling the blade to be lifted or lowered suchthat the actual tractive force detected by the actual tractive forcedetecting means is made equal to a preset target tractive force if thereis a difference between the actual tractive force and the preset targettractive force;

(e) second operation amount calculating means for calculating anoperating amount for controlling the blade to be lifted or lowered suchthat the actual position of the cutting edge detected by the actualcutting edge position detecting means is made coincident with a presettarget cutting edge position if there is a difference between the actualcutting edge position and the preset target cutting edge position;

(f) weight characteristic setting means for setting a weightcharacteristic for automatic digging, which gives importance toweighting of the operation amount calculated by the first operationamount calculating means rather than weighting of the operation amountcalculated by the second operation amount calculating means, if theloading ratio determined by the loading ratio detecting means is below aspecified value, and for setting a weight characteristic for automaticcarrying, which gives importance to weighting of the operation amountcalculated by the second operation amount calculating means rather thanweighting of the operation amount calculated by the first operationamount calculating means, if the loading ratio determined by the loadingratio detecting means is equal to or more than the specified value; and

(g) blade controlling means for controlling the blade to be lifted orlowered, using the weight characteristic set by the weightcharacteristic setting means.

According to the second aspect of the invention, if the ratio of theamount of excavated soil loaded on the front surface of the blade to itsloading capacity, which ratio is detected in dozing operation, issmaller than a specified value, a weight characteristic for automaticdigging is set. This characteristic gives importance to weighting of acontrol amount for the so-called load control (for controlling the bladeso as to make an actual tractive force equal to a target tractive force)rather than weighting of a control amount for the so-called smoothingcontrol (for controlling the blade so as to make the actual position ofthe cutting edge relative to the ground coincident with a target cuttingedge position). On the other hand, if the above loading ratio is equalto or more than the specified value, a weight characteristic forautomatic carrying is set. This characteristic gives importance toweighting of a control amount for the load control rather than weightingof a control amount for the smoothing control. With this arrangement,when operation is in the automatic digging mode, priority is given tothe load control to reduce load errors and when operation is in theautomatic carrying mode, priority is given to the smoothing control toachieve a smooth excavation face.

While switching between the weight characteristics for automatic diggingand for automatic carrying is performed according to the loading ratioin the above arrangement, it may be performed according to whether ornot the condition of the load exerted on the blade is stable.Accordingly, the second object of the invention can also be achieved bya dozing system for use in a bulldozer, according to the third aspect ofthe invention, the dozing system comprising:

(a) actual tractive force detecting means for detecting the actualtractive force of a vehicle body;

(b) cutting edge position detecting means for detecting the position ofthe cutting edge of a blade in relation to the ground;

(c) load condition detecting means for determining whether the loadexerted on the blade is in a stable state;

(d) first operation amount calculating means for calculating anoperating amount for controlling the blade to be lifted or lowered suchthat the actual tractive force detected by the actual tractive forcedetecting means is made equal to a preset target tractive force if thereis a difference between the actual tractive force and the preset targettractive force;

(e) second operation amount calculating means for calculating anoperating amount for controlling the blade to be lifted or lowered suchthat the actual position of the cutting edge detected by the actualcutting edge position detecting means is made coincident with a presettarget cutting edge position if there is a difference between the actualcutting edge position and the preset target cutting edge position;

(f) weight characteristic setting means for setting a weightcharacteristic for automatic digging, which gives importance toweighting of the operation amount calculated by the first operationamount calculating means rather than weighting of the operation amountcalculated by the second operation amount calculating means, if the loadcondition detecting means determines that the load on the blade is notin a stable state, and for setting a weight characteristic for automaticcarrying, which gives importance to weighting of the operation amountcalculated by the second operation amount calculating means rather thanweighting of the operation amount calculated by the first operationamount calculating means, if the load condition detecting meansdetermines that the load on the blade is in a stable state; and

(g) blade controlling means for controlling the blade to be lifted orlowered, using the weight characteristic set by the weightcharacteristic setting means.

According to the third aspect of the invention, if the load exerted onthe blade in dozing operation is not in a stable state, a weightcharacteristic for automatic digging is set, which characteristic givesimportance to weighting of a control amount for the load control ratherthan weighting of a control amount for the smoothing control. If theload exerted on the blade is in a stable state, a weight characteristicfor automatic carrying is set, which characteristic gives importance toweighting of a control amount for the smoothing control rather thanweighting of a control amount for the load control. Like theabove-described arrangement having the second feature, the arrangementof the third aspect is made such that when automatic digging isperformed, priority is given to the load control to reduce load errors,whereas when automatic carrying is performed, priority is given to thesmoothing control so that the face of an excavation can be flattened.

For switching between the weight characteristics, the above loadingratio and the data on whether the load exerted on the blade is stable ornot may be both used. Therefore, the second object of the invention canbe achieved by a dozing system for use in a bulldozer, according to theforth aspect of the invention, the dozing system comprising:

(a) actual tractive force detecting means for detecting the actualtractive force of a vehicle body;

(b) cutting edge position detecting means for detecting the position ofthe cutting edge of a blade in relation to the ground;

(c) loading ratio detecting means for detecting a loading ratio that isthe ratio of the amount of excavated soil loaded on the front surface ofthe blade to the loading capacity of the blade front surface;

(d) load condition detecting means for determining whether the loadexerted on the blade is in a stable state;

(e) first operation amount calculating means for calculating anoperating amount for controlling the blade to be lifted or lowered suchthat the actual tractive force detected by the actual tractive forcedetecting means is made equal to a preset target tractive force if thereis a difference between the actual tractive force and the preset targettractive force;

(f) second operation amount calculating means for calculating anoperating amount for controlling the blade to be lifted or lowered suchthat the actual position of the cutting edge detected by the actualcutting edge position detecting means is made coincident with a presettarget cutting edge position if there is a difference between the actualcutting edge position and the preset target cutting edge position;

(g) weight characteristic setting means for setting a weightcharacteristic for automatic digging, which gives importance toweighting of the operation amount calculated by the first operationamount calculating means rather than weighting of the operation amountcalculated by the second operation amount calculating means, if theloading ratio determined by the loading ratio detecting means is below aspecified value or if the load condition detecting means determines thatthe load on the blade is not in a stable state, and for setting a weightcharacteristic for automatic carrying, which gives importance toweighting of the operation amount calculated by the second operationamount calculating means rather than weighting of the operation amountcalculated by the first operation amount calculating means, if theloading ratio is equal to or more than the specified value and the loadcondition detecting means determines that the load on the blade is in astable state; and

(h) blade controlling means for controlling the blade to be lifted orlowered, using the weight characteristic set by the weightcharacteristic setting means.

According to the forth aspect of the invention, if the ratio of theamount of excavated soil loaded on the front surface of the blade to itsloading capacity, which ratio is detected in dozing operation, issmaller than a specified value or if the load exerted on the blade isnot in a stable state in dozing operation, an operation amount forautomatic digging is set, which gives importance to weighting of acontrol amount for the load control rather than weighting of a controlamount for the smoothing control. If the loading ratio is equal to ormore than the specified value and the load exerted on the blade is in astable state, a weight for automatic carrying is set, which givesimportance to weighting of a control amount for the smoothing controlrather than weighting of a control amount for the load control. Bysetting a weight characteristic for automatic carrying when therequirement for the loading ratio and the stable load condition are bothmet, the control performance of the system can be further improved.

Where the loading ratio is used for switching between the weightcharacteristics, the weight characteristics may not be classified intotwo groups, i.e., automatic digging and automatic carrying, but may beclassified into many groups according to the values of the loadingratio. Therefore, the second object can also be accomplished by a dozingsystem for use in a bulldozer, according to the fifth aspect of theinvention, the dozing system comprising:

(a) actual tractive force detecting means for detecting the actualtractive force of a vehicle body;

(b) cutting edge position detecting means for detecting the position ofthe cutting edge of a blade in relation to the ground;

(c) loading ratio detecting means for detecting a loading ratio that isthe ratio of the amount of excavated soil loaded on the front surface ofthe blade to the loading capacity of the blade front surface;

(d) first operation amount calculating means for calculating anoperating amount for controlling the blade to be lifted or lowered suchthat the actual tractive force detected by the actual tractive forcedetecting means is made equal to a preset target tractive force if thereis a difference between the actual tractive force and the preset targettractive force;

(e) second operation amount calculating means for calculating anoperating amount for controlling the blade to be lifted or lowered suchthat the actual position of the cutting edge detected by the actualcutting edge position detecting means is made coincident with a presettarget cutting edge position if there is a difference between the actualcutting edge position and the preset target cutting edge position;

(f) weight characteristic setting means for setting an adequate weightcharacteristic that is retrieved from prestored data by the loadingratio detected by the loading ratio detecting means, said prestored datacorrelating weight characteristics for operation amounts calculated bythe first operation amount calculating means and calculated by thesecond operation amount calculating means with a multiplicity of zonesinto which the value of loading ratio is stratified; and

(g) blade controlling means for controlling the blade to be lifted orlowered, using the weight characteristic set by the weightcharacteristic setting means.

According to the fifth aspect of the invention, the value of the loadingratio of the blade front surface detected in dozing operation isstratified into a multiplicity of zones and weight characteristicscorresponding to the respective zones are prestored. A weightcharacteristic corresponding to a loading ratio actually detected isretrieved from the prestored data, thereby setting an adequate weightcharacteristic. This contributes to a further improvement in controlperformance.

The third object can be accomplished by a dozing system for a bulldozeraccording to the sixth aspect of the invention, the dozing systemcomprising:

(a) cutting edge position detecting means for detecting the position ofthe cutting edge of a blade in relation to the ground;

(b) target cutting edge position setting means for setting therelationship between the actual travel distance of the bulldozer from adigging start point and target positions for the cutting edge of theblade in relation to the ground;

(c) load condition detecting means for determining whether the loadexerted on the blade is in a stable state;

(d) target cutting edge position correcting means for accumulating asequence of data on the position of the cutting edge in each dozingoperation cycle when the load condition detecting means determines theload exerted on the blade is in a stable state during dozing operationcarried out in an automatic driving mode, and for correcting the targetcutting edge position set by the target cutting edge position settingmeans to a value obtained by averaging the sequence of accumulatedcutting edge position data; and

(e) blade controlling means for controlling the blade to be lifted orlowered such that a cutting edge position detected by the cutting edgeposition detecting means is made coincident with the target cutting edgeposition corrected by the target cutting edge position correcting means.

According to the sixth aspect of the invention, if the load exerted onthe blade is stable when dozing operation is performed in an automaticdriving mode, a series of data on the position of the cutting edge areaccumulated. The target cutting edge position set in the period wherethe load is stable is corrected to a value obtained by averaging theseries of accumulated data. Based on the corrected target cutting edgeposition, control for adjusting the position of the blade cutting edge(i.e., smoothing control) is performed. Thus, the system performs dozingoperation, while learning the soil property and working conditions inthe excavation site. This arrangement enables automatic dozing operationsuited for working conditions which vary every site.

In the first, third, forth and sixth arrangements, it is preferable thatthe load condition detecting means determine that the load exerted onthe blade is stable, when a change in the load on the blade is below aspecified value and the load on the blade is proximate to a presettarget tractive force. The magnitude of a variation in the load exertedon the blade may be detected by sensing a change in the actual tractiveforce of the vehicle body or alternatively by sensing a change in theposition of the cutting edge of the blade relative to the ground.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an outside view of a bulldozer, illustrated for explaining adozing system for a bulldozer according to a first embodiment.

FIG. 2 is a skeleton diagram of a power transmission system adapted inthe dozing system for a bulldozer according to the first embodiment.

FIG. 3 is a schematic block diagram showing the system structure of thedozing system for a bulldozer according to the first embodiment.

FIG. 4 is a flow chart of the operation of the dozing system accordingto the first embodiment (the first half portion).

FIG. 5 is a flow chart of the operation of the dozing system accordingto the first embodiment (the second half portion).

FIG. 6 is a graph of an engine characteristic map.

FIG. 7 is a graph of a pump correction characteristic map.

FIG. 8 is a graph of a torque converter characteristic map.

FIG. 9 is a graph of a pitch angle vs. load correction characteristicmap.

FIG. 10 is a graph showing variations in actual tractive force withtime.

FIG. 11 is a graph of a load control characteristic map.

FIG. 12 is a graph of a leveling control characteristic map.

FIG. 13 is a graph of a load vs. weight for leveling controlcharacteristic map.

FIG. 14 is a flow chart of the important part of the operation of thedozing system according to the first embodiment.

FIG. 15 is a graph of a weight characteristic map for automatic carryingoperation.

FIG. 16 is a graph of a weight characteristic map for automatic diggingoperation.

FIG. 17 is a graph showing the relationship between actual traveldistance and loading ratio in a third embodiment.

FIG. 18 is a flow chart of the important part of the operation of thedozing system according to a forth embodiment.

FIGS. 19(a), 19(b), and 19(c) are graphs for explaining the content ofthe control performed by the dozing system according to the forthembodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the accompanying drawings, preferred embodiments of thedozing system for a bulldozer according to the invention will bedescribed.

FIRST EMBODIMENT

Referring to FIG. 1 showing an outside view of a bulldozer 1, there areprovided, on a vehicle body 2, a bonnet 3 for housing an engine (notshown) and a cab 4 for the operator who drives the bulldozer 1. Disposedon both right and left sides of the vehicle body 2 when viewed in theforward moving direction of the vehicle body 2 are crawler belts 5 (thecrawler belt on the right side is not shown in the drawing) for drivingthe vehicle body 2 so as to travel forwardly and reversely and turn. Thecrawler belts 5 are respectively independently driven by driving powertransmitted from the engine with the aid of their correspondingsprockets 6.

A blade 7 is supported on the leading ends of right and left straightframes 8, 9 the base ends of which are, in turn, pivotally supported atthe sides of the vehicle body 2 through trunnions 10 (the trunnion onthe right side is not shown in the drawing) such that the blade 7 can belifted or lowered. A pair of blade lift cylinders 11 are disposedbetween the blade 7 and the vehicle body 2, for lifting or lowering theblade 7. For tilting the blade 7 to the right and left, a brace 12 isdisposed between the blade 7 and the left straight frame 8 and a bladetilt cylinder 13 is disposed between the blade 7 and the right straightframe 9.

A steering lever 15, a gear shift lever 16 and a fuel control lever 17are disposed on the left side of the cab 4 when viewed in the traveldirection of the vehicle body 2. On the left side of the vehicle body 2,there are provided (i) a blade control lever 18 for lifting, loweringand leftwardly and rightwardly tilting the blade 7, (ii) a first dialswitch 19A for setting a value for the load of excavated soil imposed onthe blade 7, (iii) a second dial switch 19B for correcting the set loadvalue, (iv) an automatic/manual driving mode selector switch 20 forswitching on and off automatic dozing operation, (v) a lock-up selectorswitch 21 for switching on and off the locked-up state of a torqueconverter and (vi) a display unit 22. There is disposed a deceleratorpedal in front of the cab 4, although it is not shown in the drawing.

Referring to FIG. 2 which shows a power transmission system, the rotarydriving power of the engine 30 is transmitted to a damper 31 and a PTO32 for driving various hydraulic pumps including an implement operatinghydraulic pump and then to a torque converter 33 having a lock-upmechanism 33 a and a pump 33 b. The rotary driving power is thentransmitted from the output shaft of the torque converter 33 with alock-up mechanism to a transmission 34 (e.g., wet multiple disc clutchtype planetary gear transmission) which has an input shaft connected tothe output shaft of the torque converter unit 33. The transmission 34comprises a forward drive clutch 34 a, a reverse drive clutch 34 b andfirst to third speed clutches 34 c, 34 d, 34 e, so that the output shaftof the transmission 34 is rotated in three speed ranges in both forwarddrive and reverse drive. The rotary driving power from the output shaftof the transmission 34 is transmitted to paired right and left finalreduction gear mechanisms 36 through a steering system 35 to power therespective sprockets 6 for running the crawler belts 5. The steeringsystem 35 has a transverse shaft 35 e having a pinion 35 a, a bevel gear35 b, paired right and left steering clutches 35 c and steering brakes35 d. Reference numeral 37 designates an engine speed sensor fordetecting the engine speed of the engine 30 and reference numeral 38designates a torque converter output shaft revolution sensor fordetecting the revolution speed of the output shaft of the torqueconverter unit 33 with a lock-up mechanism.

As shown in FIG. 3 which schematically shows the system structure of thedozing system for a bulldozer of this embodiment, the following data areall input to a microcomputer 41 through a bus 40. (i) Dial value datathat is representative of a set value for the load of excavated soilimposed on the blade 7 and sent from the first dial switch 19A; and dialvalue data that is representative of a correction value for the set loadand sent from the second dial switch 19B. (ii) An automatic/manualdriving mode selection instruction that is representative of whether anautomatic driving mode or a manual driving mode has been selected andsent from the automatic/manual driving mode selector switch 20. (iii) Alock-up (L/U)/torque converting (T/C) selection instruction that isrepresentative of whether or not the torque converter 33 is to be lockedup and sent from the lock-up selector switch 21. (iv) Engine speed datathat is representative of the engine speed of the engine 30 and sentfrom the engine speed sensor 37. (v) Revolution data that isrepresentative of the revolution speed of the output shaft of the torqueconverter 33 and sent from the torque converter output shaft revolutionsensor 38. The following data are also input to the computer 41 throughthe bus 40. (i) Stroke positional data sent from a blade lift cylinderstroke sensor 42 for sensing the stroke positions of the right and leftblade lift cylinders 11 for lifting and lowering the blade 7. (ii) Pitchangle data sent from a pitch angle sensor 43 for sensing the momentarilychanging pitch angle of the vehicle body 2 which pitches back andforward. (iii) Speed range data sent from a transmission speed changesensor 44 for sensing which speed range in the forward and reverse drivehas been selected by shifting the transmission 34 with the gear shiftlever 16. (iv) Manual driving operation data sent from a blade operationsensor 45 for sensing whether the blade 7 has been put in manual drivingoperation by operating the blade control lever 18.

The microcomputer 41 is composed of (i) a central processing unit (CPU)41A for executing a specified program, (ii) a read only memory (ROM) 41Bfor storing the above program and various maps including an enginecharacteristic map and torque converter characteristic map, (iii) arandom access memory (RAM) 41C serving as a working memory necessary forexecuting the program and serving as various types of registers, and(iv) a timer 41D for measuring elapsed time for an event in the program.According to (i) the dial value data representative of a set value forthe load of excavated soil imposed on the blade 7 and the dial valuedata representative of a correction value for the set load, (ii) theautomatic/manual driving mode selection instruction for dozingoperation, (iii) the lock-up (L/U)/torque converting (T/C) selectioninstruction for the torque converter 33, (iv) the engine speed data onthe engine 30, (v) the revolution data on the output shaft of the torqueconverter 33, (vi) the stroke positional data on the right and leftblade lift cylinders 11, (vii) the pitch angle data on the vehicle body2, (viii) the speed range data on the transmission 34, and (ix) themanual driving operation data on the blade 7, the above program isexecuted to provide a blade lift cylinder controller 46 with a liftoperation amount to be used for lifting or lowering the blade 7.According to the lift operation amount, the right and left blade liftcylinders 11 are driven with the aid of a lift valve actuator 47 and alift cylinder control valve 48 so that the blade 7 is lifted or lowered.The display unit 22 displays whether the bulldozer 1 is presently in theautomatic driving mode or in the manual driving mode in dozingoperation.

Reference is now made to the flow charts of FIGS. 4 and 5 for explainingthe operation of the dozing system for a bulldozer having the abovestructure.

S1 to S3: An execution of the specified program is started by turning onthe electric power source, and initialization is done, for instance, forclearing the contents of the registers in the RAM 41C of themicrocomputer 41. During the period of t₁ seconds after theinitialization, pitch angle data pieces are successively read in fromthe pitch angle sensor 43 for obtaining an initial value. The reason whya sequence of data are read from the pitch angle sensor 43 is that thepitch angle of the vehicle body 2 is obtained from the frequencyseparation of the moving average of the pitch angle data.

S4 to S6: The dial value data representative of a set value for the loadof excavated soil imposed on the blade 7 is read from the first dialswitch 19A. The dial value data representative of a correction value forthe set load is read from the second dial switch 19B. An instruction forselecting the automatic or manual driving mode for dozing operation isread from the automatic/manual driving mode selector switch 20. AnL/U-T/C selection instruction for torque converter 33 is read from thelock-up selector switch 21. The engine speed data of the engine 30 isread from the engine speed sensor 37. The revolution data of the outputshaft of the torque converter 33 is read from the torque converteroutput shaft revolution sensor 38. The stroke position data of the rightand left blade lift cylinders 11 are read from the blade lift cylinderstroke sensor 42. The pitch angle data of the vehicle body 2 is readfrom the pitch angle sensor 43. The speed range data of the transmission34 is read from the transmission speed range sensor 44. The manual driveoperation state data of the blade 7 is read from the blade operationsensor 45. If supply voltage is normal, being more than a specifiedvalue and the electronic circuit and others are in their normaloperating condition, the following data processing will be carried out.

(1) Low frequency components are extracted from the sequential pitchangle data by the frequency separation of the moving average of thepitch angle data, whereby the pitch angle of the vehicle body 2 isobtained.

(2) The acceleration of the vehicle body 2 is obtained by extractingacceleration components by frequency separation in which the lowfrequency components are deducted from the sequential pitch angle data.

(3) The stroke position data pieces of the right and left blade liftcylinders 11 are averaged to obtain average stroke position data basedon which, the average of the angles of the right and left straightframes 8, 9 in relation to the vehicle body 2 is obtained as a straightframe relative angle ψ₁.

(4) From the straight frame relative angle ψ₁ and the pitch angle of thevehicle body 2 obtained in the way described in the column (1), theaverage of the angles of the right and left straight frames 8, 9relative to the ground is obtained as a straight frame absolute angle.Then, a moving average straight frame absolute angle ψ₂ is obtained fromthe moving average of the sequential data on the straight frame absoluteangle, which have been read during the period of 5 seconds.

S7 to S11: If the transmission 34 is placed in the first forward speed(F1) or the second forward speed (F2), the actual tractive force F_(R)is calculated, selecting either of the following ways according towhether the L/U-T/C selection instruction indicates the locked-up stateor torque converting state.

1. Where the torque converter 33 is in the locked-up (LU) state:

Engine torque Te is obtained from the engine characteristic map shown inFIG. 6, based on the engine speed Ne of the engine 30. Then, the enginetorque Te is multiplied by a reduction ratio k_(se) from thetransmission 34 to the final reduction mechanisms 36 through thesteering system 35 (in other words, the reduction ratio between theoutput shaft of the torque converter 33 and the sprockets 6) and furthermultiplied by the diameter r of the sprockets 6, to obtain tractiveforce Fe (=Te·k_(se)·r). A tractive force correction value Fc issubtracted from the tractive force Fe, thereby obtaining actual tractiveforce F_(R) (=Fe−Fc). The tractive force correction value Fc correspondsto the consumption of the hydraulic pumps (e.g., the implement operatinghydraulic pump working on the blade lift cylinders 11 in the PTO 32),and can be obtained from the pump correction characteristic map shown inFIG. 7, based on the lift operation amount of the blade 7.

2. Where the torque converter 33 is in the torque converting (TC) state:

A torque coefficient t_(p) and torque ratio t are obtained from thetorque converter characteristic map shown in FIG. 8, based on speedratio e (=Nt/Ne) that is the ratio of the revolution speed Nt of theoutput shaft of the torque converter 33 to the engine speed Ne of theengine 30, and then torque converter output torque Tc (=tp·(Ne/1000)²·t)is obtained. Similarly to the case 1, the torque converter output torqueTc is multiplied by the reduction ratio k_(se) between the output shaftof the torque converter 33 and the sprockets 6 and further multiplied bythe diameter r of the sprockets 6, to obtain actual tractive force F_(R)(=Tc·k_(se)·r).

Then, the load correction value, which corresponds to the pitch angle ofthe vehicle body 2 and which has been obtained from the pitch angle-loadcorrection characteristic map shown in FIG. 9, is subtracted from theactual tractive force F_(R), thereby obtaining corrected actual tractiveforce F.

S12 to S16: If the driving mode selection instruction sent from theautomatic/manual driving mode selector switch 20 indicates that theautomatic driving mode of dozing operation is selected, the followingprocessing is carried out.

1) If the length of the time during which the automatic/manual drivingmode selector switch 20 has been depressed for mode changing is t₂seconds or more, the corrected actual tractive force F is set as atarget tractive force F₀.

2) If the length of the time during which the automatic/manual drivingmode selector switch 20 has been depressed for mode changing is lessthan t₂ seconds, the set value for the load of excavated soil imposed onthe blade 7 input by the first dial switch 19A is set as a targettractive force F₀.

Then, the target tractive force F₀ is increased or decreased by theamount corresponding to the value input by the second dial switch 19Bwhich value is a correction value for the set load value input by thefirst dial switch 19A, whereby a final target tractive force F₀ isdetermined.

S17 to S19: If t₃ seconds or more have elapsed after the automaticdriving mode of dozing operation was selected in response to the drivingmode selection instruction sent from the automatic/manual driving modeselector switch 20, the moving average straight frame absolute angle ψ₂is set as a target position ψ₀ for the cutting edge of the blade 7relative to the ground. If a time less than t₃ seconds has elapsed, thestraight frame relative angel ψ₁ is set as a target position for thecutting edge of the blade 7 relative to the ground.

S20 to S23: Provided that the operation is not in the manual drivingmode, that is, the blade 7 is not manually driven by the blade controllever 18, if a change δF in the corrected actual tractive force F issmaller than a specified value F_(set) (δF<F_(set)) as shown in FIG. 10and the corrected actual tractive force F is proximate to the targettractive force F₀ (i.e., in cases where the load exerted on the blade 7is judged to be in a stable state), the target cutting edge position ψ₀is corrected to a moving average straight frame absolute angle ψ₂′ atthat time. On the other hand, if the change δF in the corrected actualtractive force F exceeds the specified value F_(set), or if thecorrected actual tractive force F differs from the target tractive forceF₀ more than a certain value (i.e., in cases where the load exerted onthe blade 7 is not in a stable state), the flow proceeds to the nextstep, without correcting the target cutting edge position ψ₀.

S24 to S25: The difference ΔF between the target tractive force F₀ andthe corrected actual tractive force F and the difference Δψ between thetarget cutting edge position ψ₀ and the moving average straight frameabsolute angle ψ₂ are obtained while the display unit 22 displays thatdozing operation is carried out in the automatic drive mode.

S26 to S28: Whether or not a shoe slip (i.e., running slip) of thevehicle body 2 has occurred is determined in the following way, based onthe moving average acceleration and the corrected actual tractive forceF. Note that the moving average acceleration is obtained from the movingaverage of the accelerations of the vehicle body 2 and the accelerationsare obtained from acceleration components extracted from the pitch angledata by frequency separation. In the following conditions, 1°≈0.0174Gand W=total weight of the bulldozer 1.

1. If either of the following conditions is satisfied, it is judged thatrunning slip has occurred

(1) moving average acceleration α<−4°

(2) moving average acceleration α<−2°, and corrected actual tractiveforce F>0.6W

2. If either of the following conditions is satisfied, it is judged thatrunning slip has occurred and then stopped.

(1) moving average acceleration α>0.1°

(2) corrected actual tractive force F>corrected actual tractive force Fat a start of running slip −0.1W

After judging whether or not a running slip has occurred based on theforegoing conditions, the program proceeds to either of the followingsteps in accordance with a result of the judgment.

1. If an occurrence of running slip is detected, a lift operation amountQ_(S) for lifting the blade 7 is obtained from a slip controlcharacteristic map (not shown) in order to eliminate the running slip byreducing the load of excavated soil imposed on the blade 7.

2. If no running slip has been detected, lift operation amounts Q₁ andQ₂ are obtained in the following way.

(1) The lift operation amount Q₁ for lifting or lowering the blade 7such that the corrected actual tractive force F is made equal to thetarget tractive force F₀ is obtained from the load controlcharacteristic map shown in FIG. 11, based on the difference ΔF betweenthe target tractive force F₀ and the corrected actual tractive force F.

(2) The lift operation amount Q₂ for lifting or lowering the blade 7such that the moving average straight frame absolute angle ψ₂ is madeequal to the target cutting edge position ψ₀ is obtained from theleveling control characteristic map shown in FIG. 12, based on thedifference Δψ between the target cutting edge position ψ₀ and the movingaverage straight frame absolute angle ψ₂.

(3) A lift operation amount Q_(T) is obtained by obtaining the sum ofthe lift operation amounts Q₁ and Q₂ which are weighted based on thetractive force difference ΔF according to the load-leveling controlweight characteristic map shown in FIG. 13. According to the weightingmap of FIG. 13, if the tractive force difference ΔF is within ±0.1W,priority is given to the load control.

If supply voltage is not normal, being equal to or lower than thespecified voltage and the electronic circuit etc. is not in a normaldriving condition, or if the transmission 34 is placed neither in thefirst forward speed (F1) nor in the second forward speed (F2), or if thedriving mode selection instruction sent from the automatic/manualdriving mode selector switch 20 indicates a selection of the manualdriving mode of dozing operation, or if the blade 7 is manually drivenby the blade control lever 18, a lift operation amount Q_(N) for liftingor lowering the blade 7 in Step S29 is obtained from a manual controlcharacteristic map (not shown), according to the operation amount of theblade control lever 18.

Then, the lift operation amounts Q_(S), Q_(T), Q_(N) are supplied to theblade lift cylinder controller 46. According to the lift operationamounts Q_(S), Q_(T), Q_(N), the blade lift cylinders 11 are driventhrough the lift valve actuator 47 and the lift cylinder control valve48, and in this way, the desired control for lifting or lowering theblade 7 is carried out.

According to the present embodiment, a target value for the smoothingcontrol is corrected so as to be equal to the level of the blade 7 whenthe load exerted on the blade 7 is in a stable state. Therefore, theblade 7 can be accurately controlled, with a load proximate to a targetvalue for the load control. With this arrangement, such an undesirableinconsistent situation can be avoided that, upon completion of carrying,the load control is so performed as to lift the blade 7 while thesmoothing control is so performed as to lower the blade 7 for thepurpose of reducing the amount of a change in the target position of thecutting edge. As a result, the face of an excavation can be flattened.

While the first embodiment is arranged such that the target cutting edgeposition ψ₀ is corrected to the moving average straight frame absoluteangle ψ₂′ obtained when the load exerted on the blade 7 comes in astable state, the straight frame absolute angle obtained when the blade7 becomes stable may be set as a target cutting edge position, insteadof using the moving average.

According to the present embodiment, a judgement as to whether a changein the load exerted on the blade 7 is small is made by determiningwhether the amount of a change δF in the corrected actual tractive forceF is lower than the specified value F_(set). This judgement may be madeby determining whether or not a change in the position of the cuttingedge is lower than a specified value. Alternatively, the judgement maybe made by determining whether the time differential of the amount ofthe change δF is lower than a preset value or whether the timedifferential of the amount of a change in the position of the cuttingedge is lower than a preset value. Any one of these judgement methodsmay be taken solely or a plurality of methods may be used incombination.

SECOND EMBODIMENT

The second embodiment does not differ from the first embodiment in termsof the construction of the bulldozer 1, the system structure and thebasic part of the flow chart associated with the operation of the dozingsystem. Therefore, the explanation of the parts common to bothembodiments will be omitted and the features inherent to the secondembodiment only will be explained in the following description (the samewill be applied to the description of the third and forth embodiments).

The second embodiment is designed such that whether the bulldozer 1 isin automatic digging operation or in automatic carrying operation isjudged according to the loading ratio of the blade and the condition ofthe load imposed on the blade. Based on the operation of the bulldozer 1determined by the judgment, the load-leveling control weightcharacteristic (see FIG. 13 in the first embodiment) is varied.

The operation of the dozing system for a bulldozer according to thesecond embodiment is described in Step S21 and forward in the flow chartof FIG. 14 which correspond to Step S21 and forward in FIG. 5. Now,referring to FIG. 14, the operation will be hereinafter explained.

S20 to S22: Provided that the operation is not in the manual drivingmode, that is, the blade 7 is not manually driven by the blade controllever 18, the difference ΔF between the target tractive force F₀ and thecorrected actual tractive force F and the difference Δψ between thetarget cutting edge position ψ₀ and the moving average straight frameabsolute angle ψ₂ are obtained, and the display unit 22 indicates thatthe dozing operation is in the automatic driving mode.

S23 to S29: Whether or not a running slip of the vehicle body 2 hasoccurred is determined, according to which either of the followingprocesses will be taken.

1. If an occurrence of running slip is detected, a lift operation amountQ_(S) for lifting the blade 7 is obtained from a slip controlcharacteristic map (not shown) in order to eliminate the running slip byreducing the load of excavated soil imposed on the blade 7.

2. If no running slip has been detected, lift operation amounts Q₁ andQ₂ are obtained in the following way.

(1) The lift operation amount Q₁ for lifting or lowering the blade 7such that the corrected actual tractive force F is made equal to thetarget tractive force F₀ is obtained from the load controlcharacteristic map shown in FIG. 11, based on the difference ΔF betweenthe target tractive force F₀ and the corrected actual tractive force F.

(2) The lift operation amount Q₂ for lifting or lowering the blade 7such that the moving average straight frame absolute angle ψ₂ is madecoincident with the target cutting edge position ψ₀ is obtained from theleveling control characteristic map shown in FIG. 12, based on thedifference Δψ between the target cutting edge position ψ₀ and the movingaverage straight frame absolute angle ψ₂.

(3) The loading ratio of the front surface of the blade 7 is detected.If the loading ratio is equal to or more than a preset value, the amountof the change δF in the corrected actual tractive force is a small valueless than the preset value F_(set), and the corrected actual tractiveforce F is proximate to the target tractive force F₀, importance isgiven to weighting of the lift operation amount Q₂ in the smoothingcontrol (leveling control) over weighting of the lift operation amountQ₁ in the load control, as shown in FIG. 15. Stated another way, if theabove conditions are met, a weight characteristic (weight function)W_(c) for automatic carrying operation for smoothing the face of anexcavation is selected, and a lift operation amount Q_(T) is obtainedbased on the tractive force difference ΔF that is weighted according tothis weight characteristic map. On the other hand, if any one of theabove conditions is not satisfied, that is, if the loading ratio is asmall value less than the preset value, or if the amount of the changeδF in the corrected actual tractive force is equal to or more than thepreset value F_(set), or if the corrected actual tractive force Fdiffers considerably from the target tractive force F₀, importance isgiven to weighting of the lift operation amount Q₁ in the load controlover weighting of the lift operation amount Q₂ in the smoothing control,as shown in FIG. 16. In other words, a weight characteristic (weightfunction) W_(D) for automatic digging operation stressed on the loadcontrol is selected and a lift operation amount Q_(T) is obtained basedon the tractive force difference ΔF that is weighted according to thisweight characteristic map.

The loading ratio is detected in the following way. First, the correctedactual tractive force F is calculated as described earlier and regardedas a horizontal reactive force F_(H) exerted on the blade 7. Then, anaxial force F_(c) exerted on the cylinder rod of the blade liftcylinders 11 is obtained and the yoke angle θ of the blade liftcylinders 11 is obtained with a yoke angle sensor. From the axial forceF_(c) and the yoke angle θ, a vertical reactive force F_(v) imposed onthe blade 7 is obtained, using the following equation.

F _(v) =F _(c)cos θ

The ratio of the vertical reactive force F_(v) to the horizontalreactive force (F_(v)/F_(H)) is calculated and then, the loading ratiocorresponding to the ratio F_(v)/F_(H) and to the pitch angle isobtained from the map.

In the second embodiment, the weight characteristic W_(c) for automaticcarrying operation is selected on condition that the requirement for theloading ratio of the blade is satisfied and the load exerted on theblade is stable (i.e., the amount of the change δF in the correctedactual tractive force F is lower than the specified value F_(set) andthe corrected actual tractive force F is proximate to the targettractive force F₀). It is also possible to select the weightcharacteristic W_(c) for automatic carrying operation when either ofthese conditions is met.

THIRD EMBODIMENT

The second embodiment uses two types of weight characteristics, that is,one for automatic digging operation and the other for automatic carryingoperation, depending on the working state of dozing operation. Incontrast with this, in the third embodiment, the relationship betweenthe actual traveling distance of the bulldozer 1 and the loading ratioof the blade as shown in FIG. 17 is taken into account and differentweight characteristics are selectively used according to which of theloading ratio zones the present loading ratio belongs to. Note that thevalue of the loading ratio is stratified into zones 1, 2, 3, 4 and 5 inthis embodiment. The weight characteristic corresponding to the detectedvalue of the loading ratio is read from the memory and a final liftoperation amount Q_(T) is determined according to the weightcharacteristic thus read.

The third embodiment can ensure more accurate blade control compared tothe second embodiment.

FORTH EMBODIMENT

In the forth embodiment, a map for correlating the actual traveldistance from a digging start point with the position of the bladecutting edge relative to the ground is prepared in every cycle of dozingoperation. In the respective cycles, stable cutting edge positions areaccumulated and averaged, thereby obtaining an optimum target value forthe smoothing control. The processes inherent to this embodiment arecarried out in Steps T1 to T4 (see FIG. 18) which replace Steps 21 toS23 of the first embodiment (see FIG. 5). The flow of these steps willbe described below.

T1: A map for correlating the actual travel distance K of the bulldozer1 with the target position of the cutting edge (i.e., target values forthe smoothing control) is initialized. This map is set, as shown in FIG.19(a), by determining target values for digging operation according tothe distance from a digging start point L₀ or alternatively bydetermining target values for carrying operation according to thedistance from a carrying start point L_(d).

T2 to T4: If the amount of the change δF in the corrected actualtractive force F is a small value that is less than the preset valueF_(set), and the corrected actual tractive force F becomes proximate tothe target tractive force F₀ (i.e., the load exerted on the bladebecomes stable), a target value for the position of the cutting edgewhen the load is in a stable state is corrected. Such corrected data areaccumulated and averaged to obtain an optimum target value. In this way,the soil properties and working conditions in the excavation site can belearned, and as a result, dozing operation can be automated so as toconform to working conditions which vary every excavation site. In FIG.19(b), A₁, A₂ and A₃ represent stable load regions. A₁′, A₂′, and A₃′ inFIG. 19(c) represent the ranges of target values corresponding to thestable load regions A₁, A₂ and A₃.

According to the first to forth embodiments, the actual tractive forceis obtained by calculation, but it may be obtained from a driving torqueamount detected by a driving torque sensor for sensing the drivingtorque of the sprockets 6. Alternatively, there may be provided abending stress sensor which senses the bending stress of the straightframes 8, 9 for supporting the blade 7 at the trunnions 10, and theactual tractive force may be obtained from the bending stress sensed bythis bending stress sensor.

While the torque converter unit 33 with a lock-up mechanism isincorporated in the power transmission system according to the foregoingembodiments, the invention may, of course, be applied to cases where atorque converter having no lock-up mechanism or a direct transmissionhaving no torque converter is used. In cases where a direct transmissionis used, the actual tractive force can be calculated in the same way asdescribed in the case of “the locked-up state” in the foregoingembodiment.

While the running slip of the vehicle body 2 is detected by extractingacceleration components from pitch angle data output from the pitchangle sensor 43 by frequency separation in the foregoing embodiments, itmay be detected from the output of the acceleration sensor, the outputindicating the acceleration condition of the vehicle body 2. It is alsopossible to detect the running slip by comparing the actual speed of thevehicle body 2 obtained from a Doppler speed meter with the travel speedof the crawler belts 5 which run the vehicle body 2.

Although a target position for the cutting edge in relation to theground is set by calculation in the foregoing embodiments, it may be setby a dial switch in the similar way to setting of a target tractiveforce.

What is claimed is:
 1. A dozing system for use in a bulldozer, thesystem comprising: (a) cutting edge position detecting means fordetecting the position of the cutting edge of a blade in relation to theground; (b) target cutting edge position setting means for setting atarget position of the cutting edge of the blade in relation to theground; (c) load condition detecting means for determining whether theload exerted on the blade is in a stable state; (d) target cutting edgeposition correcting means for correcting the target cutting edgeposition set by the target cutting edge position setting means to theactual position of the cutting edge at that time, if the load conditiondetecting means determines that the load exerted on the blade is in astable state when dozing operation is performed in an automatic diggingmode; and (e) blade controlling means for controlling the blade to belifted or lowered such that the position of the cutting edge of theblade detected by the cutting edge position detecting means is madecoincident with the target cutting edge position corrected by the targetcutting edge position correcting means.
 2. A dozing system for use in abulldozer according to claim 1, wherein the actual position of thecutting edge used for correcting the target cutting edge position by thetarget cutting edge position correcting means is obtained from a movingaverage.
 3. A dozing system for use in a bulldozer, the systemcomprising: (a) actual tractive force detecting means for detecting theactual tractive force of a vehicle body; (b) cutting edge positiondetecting means for detecting the position of the cutting edge of ablade in relation to the ground; (c) loading ratio detecting means fordetecting a loading ratio that is the ratio of the amount of excavatedsoil loaded on the front surface of the blade to the loading capacity ofthe blade front surface; (d) first operation amount calculating meansfor calculating an operating amount for controlling the blade to belifted or lowered such that the actual tractive force detected by theactual tractive force detecting means is made equal to a preset targettractive force if there is a difference between the actual tractiveforce and the preset target tractive force; (e) second operation amountcalculating means for calculating an operating amount for controllingthe blade to be lifted or lowered such that the actual position of thecutting edge detected by the actual cutting edge position detectingmeans is made coincident with a preset target cutting edge position ifthere is a difference between the actual cutting edge position and thepreset target cutting edge position; (f) weight characteristic settingmeans for setting a weight characteristic for automatic digging, whichgives importance to weighting of the operation amount calculated by thefirst operation amount calculating means rather than weighting of theoperation amount calculated by the second operation amount calculatingmeans, if the loading ratio determined by the loading ratio detectingmeans is below a specified value, and for setting a weightcharacteristic for automatic carrying, which gives importance toweighting of the operation amount calculated by the second operationamount calculating means rather than weighting of the operation amountcalculated by the first operation amount calculating means, if theloading ratio determined by the loading ratio detecting means is equalto or more than the specified value; and (g) blade controlling means forcontrolling the blade to be lifted or lowered, using the weightcharacteristic set by the weight characteristic setting means.
 4. Adozing system for use in a bulldozer, the system comprising: (a) actualtractive force detecting means for detecting the actual tractive forceof a vehicle body; (b) cutting edge position detecting means fordetecting the position of the cutting edge of a blade in relation to theground; (c) load condition detecting means for determining whether theload exerted on the blade is in a stable state; (d) first operationamount calculating means for calculating an operating amount forcontrolling the blade to be lifted or lowered such that the actualtractive force detected by the actual tractive force detecting means ismade equal to a preset target tractive force if there is a differencebetween the actual tractive force and the preset target tractive force;(e) second operation amount calculating means for calculating anoperating amount for controlling the blade to be lifted or lowered suchthat the actual position of the cutting edge detected by the actualcutting edge position detecting means is made coincident with a presettarget cutting edge position if there is a difference between the actualcutting edge position and the preset target cutting edge position; (f)weight characteristic setting means for setting a weight characteristicfor automatic digging, which gives importance to weighting of theoperation amount calculated by the first operation amount calculatingmeans rather than weighting of the operation amount calculated by thesecond operation amount calculating means, if the load conditiondetecting means determines that the load on the blade is not in a stablestate, and for setting a weight characteristic for automatic carrying,which gives importance to weighting of the operation amount calculatedby the second operation amount calculating means rather than weightingof the operation amount calculated by the first operation amountcalculating means, if the load condition detecting means determines thatthe load on the blade is in a stable state; and (g) blade controllingmeans for controlling the blade to be lifted or lowered, using theweight characteristic set by the weight characteristic setting means. 5.A dozing system for use in a bulldozer, the system comprising: (a)actual tractive force detecting means for detecting the actual tractiveforce of a vehicle body; (b) cutting edge position detecting means fordetecting the position of the cutting edge of a blade in relation to theground; (c) loading ratio detecting means for detecting a loading ratiothat is the ratio of the amount of excavated soil loaded on the frontsurface of the blade to the loading capacity of the blade front surface;(d) load condition detecting means for determining whether the loadexerted on the blade is in a stable state; (e) first operation amountcalculating means for calculating an operating amount for controllingthe blade to be lifted or lowered such that the actual tractive forcedetected by the actual tractive force detecting means is made equal to apreset target tractive force if there is a difference between the actualtractive force and the preset target tractive force; (f) secondoperation amount calculating means for calculating an operating amountfor controlling the blade to be lifted or lowered such that the actualposition of the cutting edge detected by the actual cutting edgeposition detecting means is made coincident with a preset target cuttingedge position if there is a difference between the actual cutting edgeposition and the preset target cutting edge position; (g) weightcharacteristic setting means for setting a weight characteristic forautomatic digging, which gives importance to weighting of the operationamount calculated by the first operation amount calculating means ratherthan weighting of the operation amount calculated by the secondoperation amount calculating means, if the loading ratio determined bythe loading ratio detecting means is below a specified value or if theload condition detecting means determines that the load on the blade isnot in a stable state, and for setting a weight characteristic forautomatic carrying, which gives importance to weighting of the operationamount calculated by the second operation amount calculating meansrather than weighting of the operation amount calculated by the firstoperation amount calculating means, if the loading ratio is equal to ormore than the specified value and the load condition detecting meansdetermines that the load on the blade is in a stable state; and (h)blade controlling means for controlling the blade to be lifted orlowered, using the weight characteristic set by the weightcharacteristic setting means.
 6. A dozing system for use in a bulldozer,the system comprising: (a) actual tractive force detecting means fordetecting the actual tractive force of a vehicle body; (b) cutting edgeposition detecting means for detecting the position of the cutting edgeof a blade in relation to the ground; (c) loading ratio detecting meansfor detecting a loading ratio that is the ratio of the amount ofexcavated soil loaded on the front surface of the blade to the loadingcapacity of the blade front surface; (d) first operation amountcalculating means for calculating an operating amount for controllingthe blade to be lifted or lowered such that the actual tractive forcedetected by the actual tractive force detecting means is made equal to apreset target tractive force if there is a difference between the actualtractive force and the preset target tractive force; (e) secondoperation amount calculating means for calculating an operating amountfor controlling the blade to be lifted or lowered such that the actualposition of the cutting edge detected by the actual cutting edgeposition detecting means is made coincident with a preset target cuttingedge position if there is a difference between the actual cutting edgeposition and the preset target cutting edge position; (f) weightcharacteristic setting means for setting an adequate weightcharacteristic that is retrieved from prestored data by the loadingratio detected by the loading ratio detecting means, said prestored datacorrelating weight characteristics for operation amounts calculated bythe first operation amount calculating means and calculated by thesecond operation amount calculating means with a multiplicity of zonesinto which the value of loading ratio is stratified; and (g) bladecontrolling means for controlling the blade to be lifted or lowered,using the weight characteristic set by the weight characteristic settingmeans.
 7. A dozing system for use in a bulldozer, the system comprising:(a) cutting edge position detecting means for detecting the position ofthe cutting edge of a blade in relation to the ground; (b) targetcutting edge position setting means for setting the relationship betweenthe actual travel distance of the bulldozer from a digging start pointand target positions for the cutting edge of the blade in relation tothe ground; (c) load condition detecting means for determining whetherthe load exerted on the blade is in a stable state; (d) target cuttingedge position correcting means for accumulating a sequence of data onthe position of the cutting edge in each dozing operation cycle when theload condition detecting means determines the load exerted on the bladeis in a stable state during dozing operation carried out in an automaticdriving mode, and for correcting the target cutting edge position set bythe target cutting edge position setting means to a value obtained byaveraging the sequence of accumulated cutting edge position data; and(e) blade controlling means for controlling the blade to be lifted orlowered such that a cutting edge position detected by the cutting edgeposition detecting means is made coincident with the target cutting edgeposition corrected by the target cutting edge position correcting means.8. A dozing system for a bulldozer according to claim 1, 4, 5, or 7,wherein the load condition detecting means determine that the loadexerted on the blade is in a stable state, when a change in the load onthe blade is below a specified value and the load on the blade isproximate to a preset target tractive force.
 9. A dozing system for abulldozer according to claim 8, wherein the magnitude of a change in theload exerted on the blade is determined by sensing a change in theactual tractive force of the vehicle body.
 10. A dozing system for abulldozer according to claim 8, wherein the magnitude of a change in theload exerted on the blade is determined by sensing a change in theposition of the cutting edge of the blade relative to the ground.